1. Field of the Invention
The present invention relates to a semiconductor producing apparatus for forming thin films on wafer surfaces using reaction gasses, and also to a wafer vacuum chucking device, a gas cleaning method or a method of forming various kinds of nitride films in such semiconductor producing apparatus. The semiconductor producing apparatus of the present invention is most effectively used with the Low Pressure CVD (Chemical Vapor Deposition) devices, but also can be used with other types of semiconductor producing devices for forming thin films, processings and the like.
2. Description of the Background Art
A conventional semiconductor producing apparatuses, and a wafer vacuum chucking device, a gas cleaning method and a nitride film forming method in the semiconductor producing apparatus will be respectively described below.
First, FIG. 54 is a top view showing the structure of a conventional wafer vacuum chucking device described in Japanese Patent Laying-Open No.2-67745, for example, and FIG. 55 is a sectional view taken along the VI--VI line in FIG. 54. In FIG. 54, 101 denotes a vacuum chuck main body, 1 denotes a wafer to be sucked and fixed (hereinafter, the term "wafer" implies a semiconductor wafer except extra notation), and 103 denotes a vacuum suction groove formed on a wafer suction surface and communicating as one groove in the suction surface, 104 denotes a vacuum evacuation hole formed at the center of the suction surface, and 105 denotes a vacuum evacuation path opening to the vacuum evacuation hole 104 and passing through the chuck main body 101, which is connected to a vacuum pump not shown through a vacuum pipe 106.
In such a conventional wafer vacuum chucking device formed as described above, the wafer 1 is provided on the suction surface of the chuck main body 101 and evacuation in the vacuum evacuation groove 103 is started through the vacuum pipe 106, the vacuum evacuation path 105 and the vacuum evacuation hole 104, then a differential pressure is caused between the pressure inside the vacuum evacuation groove 103 and the external pressure, and the wafer 1 is sucked and fixed on the suction surface with the differential pressure.
Next, FIGS. 56 and 57 are diagrams showing a semiconductor producing apparatus (so called, the batch processing type) implementing the method of forming silicon nitride films described in Japanese Patent Publication No. 60-10108, for example. In FIGS. 56 and 57, 2 denotes a tube-shaped reaction chamber, 4 denotes a wafer heating source, and 501 and 502 denote reaction gas supplying lines.
In the thin film forming method using the apparatus, a plurality of wafers 1 to be processed are arranged and held in the tube-shaped reaction chamber 2, and dichlorosilane (SiH.sub.2 Cl.sub.2) and ammonia (NH.sub.3) are caused to flow from the reaction gas supplying lines 501 and 502 under low pressure to deposit silicon nitride films, where the deposition temperature used is in the range of 650-800.degree. C.
In the above-described method, in order to surely keep the uniformity of film thickness of the plurality of wafers 1, the temperature distribution in the tube-shaped reaction chamber 2 is controlled so that the temperature of the wafers 1 increases from the upper stream to the lower stream of the gas to compensate for a decrease in the deposition rate due to consumption of the reaction gas. Also, in order to surely keep the uniformity of film thickness in the surface of wafer 1, the pressure in the reaction chamber 2 is kept low, that is, about 0.5 Torr.
Next, FIG. 59 shows a semiconductor producing apparatus described in the Japanese Patent Laying-Open No. 54-160172, the Japanese Patent Laying-Open No. 3-291381 and the like, for example. In FIG. 59, 701 denotes a reaction container providing a cylindrical inner tube 702 therein, which forms a reaction space 3 inside thereof. 704 denotes a wafer holding board arranged in the reaction space 3, which has structure for holding a large number of semiconductor wafers 1. Heater 4 is a wafer heating source for heating the semiconductor wafers 1, which is provided around the peripheral wall of the reaction container 701.
501 denotes a reaction gas line for introducing dichlorosilane which is a material gas into the reaction space 3, and 502 is a material gas supplying line for introducing ammonia which is another reaction gas into the reaction space 3. These reaction gas supplying lines 501 and 502 are provided passing through the reaction container 701, respectively, having the upstream side where the gas flows from connected to material gas supplying sources (not shown) and the downstream side to which the gas flows opening to the lower part of the reaction chamber 2.
7 denotes a reaction exhaust pass for exhausting the gas in the reaction container 701. The reaction exhaust pass 7 has its one end opening to a space formed on the outer peripheral side of the inner tube 702 inside the reaction container 701 and the other end connected to an exhaust device not shown in the figure. The pressure in the reaction container 701 is generally reduced by exhausting the internal gas through the reaction exhaust pass 7.
In the semiconductor producing apparatus having such structure, first, the semiconductor wafers 1 are heated through the peripheral wall of the reaction container 701 by the heater 4. The temperature at this time is approximately 700.degree. C. Subsequently, dichlorosilane and ammonia are introduced in the reaction space 3 from the separated reaction gas supplying lines 501 and 502. The two kinds of reaction gasses supplied into the reaction space 3 in this way are thermally decomposed in the vapor phase in contact with the heated semiconductor wafers 1. Then, the product produced by this reaction is deposited on the semiconductor wafers 705 to form silicon nitride films.
Next, FIG. 60 is a sectional diagram showing an outline of a conventional semiconductor device (so called the single wafer processing type) described in the Japanese Patent Laying-Open No. 3-184327, for example. In FIG. 60, 1 denotes a wafer, 2 denotes a reaction chamber for accommodating the wafer 1, 5 denotes a vacuum chuck on which the wafer 1 is provided, 204 denotes a vacuum drawing hole opening to the vacuum chuck 5, 4 denotes a wafer heating source provided in the vacuum chuck 5 for heating the wafer 1, 6 denotes a gas nozzle for supplying reaction gas into the reaction chamber 2, 3 denotes a reaction space in which the reaction is caused, and 7 denotes a reaction gas exhaust path for exhausting the gas in the reaction chamber 2 after the reaction.
For forming a thin film in such a structure, first the wafer 1 is conveyed by a conveying device (not shown) and provided on the vacuum chuck 5. Next, vacuum exhaust is performed from the vacuum drawing hole 204 opening to the vacuum chuck 5 to suck the wafer 1. The reaction gas is supplied into the reaction chamber 2 from the gas nozzle 6. At this time the wafer 1 is heated through the vacuum chuck 5 by the wafer heating source 4, so that the reaction gas is caused to react on the wafer 1 to form a thin film on the wafer 1.
Now, in such processes, the wafer 1 must be heated to a high temperature such as 600-800.degree. C. Also, since the quality and the growth rate of the reaction product film depend on the temperature of the wafer 1, the wafer 1 must be uniformly heated to a predetermined temperature in order to form the reaction product film with uniform quality and thickness. Furthermore, contamination to the wafer 1 and the thin film formed on the surface of the wafer 1 must be avoided.
Next, FIG. 61 is a sectional diagram showing structure of a gas seal portion of a reaction chamber in the conventional semiconductor producing apparatus (low pressure CVD apparatus) referred in the Japanese Patent Laying-Open No. 2-143526, for example. In FIG. 61, 305 denotes an o-ring, 317 denotes a process tube, 318 denotes a cap, 319 denotes a manifold, 320 denotes an inactive gas supplying hole for introducing inactive gas (N.sub.2)into the gap portion between the cap 318 and the manifold 319 and the gap porion between the process tube 317, the cap 318 and the manifold 319, and 306 denotes a water cooling portion formed in the manifold 319.
In the above-described structure, the temperature in the process tube 317 is kept high, and thermal chemical reaction of the introduced reaction gas is caused on the wafer (not shown) in the process tube 317 to form thin films. At this time the o-rings 305 serve as gas seals from outside, and water is passed to the water cooling portions 306 to cool and protect the o-rings 305 to prevent degradation of the sealing function and damage due to heat. As a result, the temperature in the vicinity of the o-ring 305 decreases, but invasion of the reactive gas into the vicinity of the o-ring 305 is prevented by introducing the inactive gas into the gap portion to prevent adhesion of reaction by-product.
Next, the conventional gas cleaning method will be described referring to FIG. 62. Generally, in a semiconductor producing apparatus, thin film material deposits or attach not only on the wafers but also on wall surfaces of the apparatus and members such as a susceptor exposed in the reaction chamber, so that these are removed by causing etching gas into the reaction chamber as shown in the Japanese Patent Laying-Open No. 3-41199. FIG. 62 shows the processing temperature for removing surplus deposition substances without producing undesirable effects on the carbon material forming the susceptor and the like in this gas cleaning method, where it is indicated that the temperature of 200-300.degree. C. is suitable.
Next, referring to FIG. 63, the structure of supplying reaction gas in the conventional semiconductor producing apparatus will be described. Conventionally, when forming a thin film for semiconductor with the single wafer processing type low pressure CVD, as shown in the Japanese Patent Laying-Open No. 3-287770, for example, a nozzle head with structure simply having a large number of through holes has been used to make the flow of reaction gas uniform to form a thin film having uniform thickness.
For example, the case in which two kinds of reaction gases are lead to a heated wafer and decomposition reaction is caused to form a film is shown in FIG. 63. In FIG. 63, 51 denotes a susceptor, 604 denotes first reaction gas, 605 denotes second reaction gas, 606 denotes a mixture gas transfer path, 6 denotes a gas nozzle, 3 denotes a reaction space and 7 denotes a reaction gas exhaust path.
In this apparatus, for example, the first reaction gas 604 and the second reaction gas 605 are led to the mixed gas transfer path 606 by means of separated pipings and sprayed toward the wafer 1 through the large number of through holes of the gas nozzle 6. The wafer 1 is provided on the susceptor 5 heated by the wafer heating source 4 and the supplied reactive gases are subjected to the decomposition reaction above the wafer 1 and then exhausted from the reaction gas exhaust path 7 and processed. The gas nozzle 6 may be provided so that the through holes face perpendicularly to the wafer 1, or may be provided inclining by 20-45.degree. with respect to the wafer 1 as shown in FIG. 63.
The wafer vacuum chucking device shown in FIGS. 54 and 55 utilizes the differential pressure as described above, in which it is assumed to be used under the normal atmospheric pressure, and chucking under low pressure is not considered in which the differential pressure is difficult to be obtained. Accordingly, when sucking to fix a wafer under low pressure with its processed surface being faced downwards, the degree of vacuum in the entire vacuum suction groove 103 which communicates as one in the entire suction surface decreases if the airtight condition in the vicinity of the wafer is broken due to adhesion of dust on the suction 'surface, cuts on the suction surface, curves of the wafers and the like, and, furthermore, since the pressure in the exterior is reduced, sufficient differential pressure can not be obtained, resulting in a problem that the wafer may drop or may not be sucked at the first stage.
Next, in the nitride film forming method using the conventional batch processing semiconductor producing apparatus described referring to FIGS. 56 and 57, or FIG. 59, there has been a problem as will be described below.
First the by-products (mainly ammonium chloride) produced in the reaction of dichlorosilane and ammonia may cause dust, resulting in a problem that the productivity of the semiconductor chips is considerably decreased.
Also, the low pressure atmosphere must have been realized with pressure lower than 1.0 Torr in the reaction chamber 2 to suppress production of the ammonium chloride, and dichlorosilane and ammonia must have been mixed in that atmosphere. Furthermore, the wall of the reaction chamber 2 must have been maintained at a high temperature (not less than 200.degree. C.) to prevent adhesion of the ammonium chloride. That is to say, the process in forming films is limited, which causes a problem that films can not be formed with appropriate device structure under optimum process conditions.
Also, in forming capacitor films for semiconductor or the like where the capacitor capacitance is secured in a small area, the film thickness is extremely small and it is required to form films having complicated cubic shapes with uniform film thickness because the surface area of the capacitor must be secured. Furthermore, to obtain a larger number of semiconductors from a single wafer to decrease cost of the semiconductor, the diameter of the wafer is on an increase. When processing wafers with large diameter by the method described in the Japanese Patent Publication No. 60-10108 with FIGS. 56, 57 and the like, however, there is a problem that the variation in the film thickness distribution in wafer increases.
Also, in order to realize film thickness with good reproducibility, the balance between the wafer temperature and the amount of reaction gas must be controlled. In the method shown in the Japanese Patent Publication No. 60-10108 with FIGS. 56, 57, i.e., in the method using the batch processing CVD apparatus, although the temperature distribution in the tube-shaped reaction chamber 2 is controlled so that the. wafer temperature increases from the upstream side to the downstream side of the gas to suppress variation of film thickness of a plurality of wafers 1, where the accuracy has a limitation. So, the number of wafers 1 processed at the same time is decreased and wafers 1 called dummy wafers are provided instead in the actual process, but it results in a problem of incurring a decrease of yield, that is, an increase in cost.
Furthermore, the properties of the underlying part in formation of the capacitor film must be accurately controlled to form extremely thin films with good reproducibility, but the method described in the Japanese Patent Publication No. 60-10108 (so called batch processing method) in which a plurality of wafers 1 are collectively processed at the same time had a problem that such control can not be made.
On the basis of such background, the CVD apparatus called the single wafer processing type (such as one described in the Japanese Patent Laying-Open No. 3-184327) described before referring to FIG. 60 has been developed there years. When using the single wafer processing CVD apparatus, however, there has been a problem that the pressure in the reaction chamber 2 must be increased to several Torr-several tens Torr from the conventional value 0.5 Torr to secure the productivity similar to the case using the conventional batch processing CVD apparatus.
The ammonium chloride produced when dichiorosilane or titanium tetrachloride and ammonia are used as reaction gases has such saturated vapor pressure characteristics. as shown in FIG. 58, that the ammonium chloride is solidified and attached unless the temperature of the wall of the reaction chamber is maintained at the high temperature as described above. The temperature of the wall of about 150.degree. C. was satisfactory with the low pressure in the reaction chamber being 0.5 Torr. However, there have been problems such as that the wall temperature must be increased to 250.degree. C. to inhibit the solidification of ammonium chloride if the pressure in the reaction chamber is increased to value from several Torr to several tens Torr, with the result that a vacuum seal member (o-ring) made of rubber generally used to maintain the vacuum state can not be used, and that the films can not be formed under the most suitable precess conditions.
Also, since the structure of the single wafer processing CVD apparatus is more complicated than that of the batch processing CVD apparatus, it is much in need to use metal materials such as SUS. However, the metal materials generally corrode more possibly due to chlorine at high temperature, causing a problem that it is limited to expensive metal materials with high corrosion resistance such as Inconel and glass materials such as quartz.
Also, in the conventional single wafer processing semiconductor producing apparatus shown in FIG. 60, since the high temperature portion of the vacuum chuck 5 is exposed to the reaction gas, a film is attached to the high temperature portion to cause occurrence of dust. If the reaction gas is corrosive gas, the wafer heating source 4 with high temperature corrodes to cause a problem. Furthermore, since the high temperature portion heated by the wafer heating source 4 and the reaction space 3 in which the wafer 1 is provided and reaction is executed are the same, there has been a problem that the wafer 1 and the thin film formed on the wafer surface may be contaminated from the wafer heating source 4 and the high temperature portion in the vicinity thereof.
Also, the wafer holding mechanism is the vacuum chuck 5, so that a vacuum layer is formed in the space between the wafer 1 and the wafer holding mechanism to considerably increase the thermal resistance between both, with the result that the temperature of the wafer heating source 4 must be increased to heat the wafer 1 to a high temperature (600 to 800.degree. C., for example) when forming a film. Furthermore, there has been a problem that the temperature distribution on the wafer 1 could not be made uniform if there were variations in the distribution of generation of heat by the wafer heating source 4.
Next, in the conventional semiconductor producing apparatus having the gas seal portion shown in FIG. 61, the effect of diffusion of reactive gas is large under low pressure such as several Torr as shown in the Japanese Patent Laying-Open No. 2-143526, therefore a large amount of inactive gas must be introduced to inhibit the diffusion and prevent invasion of reactive gas. Furthermore, there has been a problem that the inactive gas is introduced into the reaction chamber through a narrow gap portion in that structure, so that it will be jetted out into the reaction chamber even if a small amount of reaction by-products is attached.
Next, the conventional gas cleaning method described referring to FIG. 62 has a problem that even a surface of the susceptor on which the surplus depositions are not attached may be etched. The temperature of the susceptor generally increases to the second highest in the semiconductor producing apparatus, where that of the heater which is a heating source is the highest, and carbon coated with SiC is often used as material of the susceptor. The temperature in the apparatus is increased in cleaning to decrease the cleaning time, and it is a matter of course that the temperature of the susceptor also increases in this case. However the susceptor has some surface on which no surplus deposition is attached because they are in contact with the wafer and also has some part on which a large amount of surplus depositions are attached right in the vicinity thereof. Accordingly, there has been a problem that the surface having no surplus depositions attached thereon may be etched if the temperature is increased to 200-300.degree. C. or higher as described before to reduce the cleaning time.
Next, the structure of supplying reaction gas shown in FIG. 63 has a problem that, when a silicon nitride film is formed using SiH.sub.2 Cl.sub.2 and NH.sub.3 as reaction gases, these reaction gases produce the by-product NH.sub.4 Cl (white solid substance) even if they are mixed at room temperature, with a result that a portion in which the two kinds of reactive gases are mixed, especially the outlet portion of the piping for SiH.sub.2 Cl.sub.2 connected to the mixture transfer path 606 may be clogged and that foreign substances may attach on the wafer 1.
Also, if the two kinds of reaction gases are mixed and the reaction gas is supplied onto the wafer 1 through the gas nozzle 6 to form a thin film, there has been a problem in the uniformity of film thickness. For example, the data example of the film thickness distribution in 6-inch wafer surface formed with the above-described apparatus is shown in FIG. 64. In the film forming conditions, the total pressure was 30 Torr, the SiH.sub.2 Cl.sub.2 flow rate/NH.sub.3 flow rate diluted with N.sub.2 flow were respectively 10 sccm/40 sccm diluted with 550 sccm, and the wafer surface temperature was 700.degree. C. The data shows pattern of the film thickness distribution inclining on one side, where the film thickness on the side closer to the piping for SiH.sub.2 Cl.sub.2 is especially large. This indicates a problem that the reaction gases SiH.sub.2 Cl.sub.2 and NH.sub.3 are not mixed uniformly enough.